Plasma-Enhanced Atomic Layer Deposition of Superconducting NbN Films

• NbN is a Type II superconductor with a Tc of about 16K1

• Superconducting NbN has been deposited by Plasma-Enhanced Atomic Layer Deposition (PEALD) previously.2

• Tc = 10.2K, Room T resistivity = 250-cm

• PEALD NbN has been studied for gate electrode applications: • Nb(N-t-Bu)(NMeEt)3 + H2 plasma, 715-cm3

• Nb(N-t-Bu)(NMeEt)34

• Improvements in PEALD NbN process may lead to higher Tc and lower resistivity values

• PEALD of NbN films on an Ultratech/CNT Fiji • (t-butylimido) tris(diethylamido) niobium(V) (TBTDEN) (100°C)

• Spectroscopic ellipsometry for film thickness, n, and k • Four point probe technique to assess thin film resistivity • Tc measured with Quantum Design PPMS through a Stanford

Research Systems SR830 lock-in amplifier • Composition data from PHI Versaprobe XPS

Resistivity and Superconductivity

Mark J. Sowaa, Yonas Yemaneb, Fritz Prinzb, J Provineb

• Superconductive PEALD NbN with critical temperature, Tc, values as high as 13.5K

• Room-temperature resistivity values as low as 173-cm

Experimental

Introduction

Highlights

Growth Per Cycle

a. Ultratech-CNT b. Stanford University

Composition

• GPC primarily depends on temperature • Small GPC increase with H2 flow at 300°C/300W • Small GPC decrease over power range at 80sccm H2/300°C

References: 1. B. T. Matthias, et al., Rev. Mod. Phys. 35, 1 (1963). 2. M. Ziegler, et al., Supercond. Sci. Technol. 26 (2013) 025008. 3. J. Hinz, et al., Semicond. Sci. Technol., Volume 25, 075009, 2010.

• Binding energy of the Nb 3d5/2 peak is ~203.5 eV for all samples consistent with NbN (203.5 - 204 eV) or NbO (202.8 - 204.8 eV) but not Nb metal (201.8 - 202.5 eV)5

• C1s peak at ~282.5 eV suggests presence of NbC5

• Depositions resulted in Nb-rich films • H-rich plasmas deplete film of N resulting in higher Nb:N • Temperature increase increases Nb and decreases N resulting

in higher Nb:N

Optical Properties

Conclusions • Superconductive PEALD NbN with Tc values as high as 13.5K • Room-temperature resistivity values as low as 173-cm • Resistivity and n decrease, Tc and k increase as NbN films

become more Nb-rich at high deposition temperature, high H2 flow rate, and high plasma power.

• Refractive index decreases and extinction coefficient increases depend primarily on temperature and H2 flow rate increases

• Plasma power increase is a secondary influence on decreasing n and increasing k

• Higher k and lower n in Nb-rich films

Acknowledgements • The authors would like to thank Dr. Jinsong Zhang from Prof. Yi Cui's group at

Stanford University for helping us with the superconductivity measurements.

500 cycles TBTDEN Pulse TBTDEN Purge Plasma Plasma Purge

Carrier Ar (sccm) 30 30 10 30

Plasma Ar (sccm) 100 100 0 100

Plasma H2 (sccm) 0 0 0—80 0

Plasma N2 (sccm) 5 5 5 5

Pump Speed Low High High Low

Power (W) 0 0 150-300 0

Time (sec) 3x BoostTM 8 40 5

Temperature (°C) 100 – 300

• Resistivity decrease and Tc increase depend primarily on temperature and H2 flow rate increases

• Plasma power increase is a secondary influence on decreasing resistivity and increasing Tc

• Nb-rich films have lower resistivity and higher Tc

• Negative linear relationship between Tc and resistivity predicts 100C (3035-cm) and 150C (772-cm) films not superconducting

4. J. Hinz, et al., Semicond. Sci. Technol., Volume 25, 045009, 2010. 5. J. F. Moulder, Handbook of X-ray Photoelectron Spectroscopy,

edited by J. Chastain (Perkin-Elmer Corporation, 1992).